For a wheel rim (for, e.g., a truck or bus), a material selected from at least two kinds of materials including a flat-plate material 1 as illustrated in FIG. 5 and a die steel 2 having a shaped configuration as illustrated in FIG. 6 can be used. Further, a material 3 having a non-uniform thickness (as illustrated in FIGS. 7 and 8) disclosed in Japanese Patent Publication No. HEI 8-91005 can be used.
Rim manufacturing processes from those materials are shown in FIGS. 9A, 9B, and 9C. In the manufacturing processes, as illustrated in FIG. 9A, the flat-plate material is formed to a rim configuration through a coiling step 4, a welding step 5, a trimming step 6, a flaring or expanding step, and a roll-forming step 7. As illustrated in FIG. 9C, the die steel is formed to a rim configuration through a coiling step, a welding step, and then an expanding step in a radial direction 8 (sizing), because the die steel has a cross-section close to a final configuration of the rim. As illustrated in FIG. 9B, a material having a non-uniform thickness of Publication 8-91005 may be formed to a rim configuration through the steps 4, 5, 7 and 8 (the trimming step 6 is not used).
Trimming of a welded burr conducted after rim-welding is executed as follows: In the case of a cylindrical material manufactured from the flat-plate material, as illustrated in FIGS. 10A and 10B, a welded burr is trimmed by moving a cutter 9 in an axial direction of the cylindrical material. In the case of an annular material manufactured from the die steel, as illustrated in FIGS. 11A and 11B, a burr of a welded portion W is trimmed by moving a cutter 10, which has the same configuration as that of a cross section of an annular material having a convex and/or concave configuration, in a circumferential direction of the annular material. In the case of an annular material manufactured from the material having a non-uniform thickness of Publication 8-91005, trimming is conducted in the same way as in the die steel.
The above-described conventional rim manufacturing processes have, for example, the following problems:
With cylindrical material manufactured from the flat-plate material, the cylindrical material has a uniform thickness except that a local thickness reduction occurs due to coiling and forming. Therefore, a thickness distribution in proportion to a stress distribution cannot be expected, and the thickness of the cylindrical material is uniformly thick in order to satisfy a required thickness of portions where rigidity and fatigue strength are most needed. As a result, the material is heavy and expensive, and it is difficult to obtain lightening and cost requirements.
Annular material manufactured from a die steel has a substantially constant thickness. Annular material has a large wave and a convex and/or concave configuration in the axial direction of the rim. Distortion and interference are likely to occur during the coiling, welding and forming steps. The appearance of the rim is also inferior Further, the welded portions can be trimmed only by moving the cutter in a circumferential direction of the rim as illustrated in FIG. 11A and FIG. 11B. Machining to a smooth surface is also difficult. As a result, the appearance and design quality is low, and additional machining for improving the appearance after trimming is usually required. Even if such additional machining is performed, the appearance is inferior, and cost control becomes difficult.
Japanese Patent Publication HEI 8-91005 (FIG. 7) proposed a method for manufacturing a rim from a material having various thicknesses at various portions of the material.
In Japanese Patent Publication HEI 8-91005, as illustrated in FIG. 7 and FIG. 8, thick portions of the material correspond to corner portions (axially curved portions) of the rim, so that the final configuration has a surface shaped in a convex arc. In the case of an arc-shaped surface, if a thick portion where maximum strength is required is offset in a width direction (i.e., an axial direction) of the rim, a required thickness cannot be assured at an objective portion. This means that a required strength cannot be assured at the objective portion, and a crack could be caused at the objective portion. To prevent this, an extra thickness must be added to the objective portion.
In addition, when a material is conveyed on conveyer rolls while being rolled, if the material has a flat surface and is thin, a front end portion of the conveyed material will be bent downward between the rolls due to the weight of the material, and thus, a smooth conveyance will be difficult.
Further, during the step of coiling, if the material has a flat surface, a pinch force caused by the pinch rolls in order to feed the material to coiling rolls is liable to be insufficient and problems may arise in feeding the material to the coiling rolls. If the clearance between the pinch rolls is decreased so that the material is forcibly fed to the coiling rolls, slippage is liable to occur between the coiling rolls and the material, and the material could be scuffed.
Further, since the surface of the thick portion of the curved portion of the rim has the configuration of an arc, as illustrated in FIG. 8, the surface of the curved portion includes a steep, inclined portion 23 which has a large inclination angle exceeding, for example, 30 degrees. In such a case, in the trimming step (6) of FIG. 9B, axial trimming is impossible, and only circumferential trimming can be used. However, as illustrated in FIGS. 12A, 12B, 12C and 12D, circumferential trimming is accompanied by the generation of an overlap 22, which is formed when a remaining material 21 of a removal material 20 is pushed by rolls in a succeeding step (for example, the roll-forming step). More particularly, when the welded portion W of the annular material is trimmed in the circumferential direction C as illustrated in FIG. 12A, by moving the cutter 10 as illustrated in FIG. 12B thereby removing the burr (removal material) 20, the remaining material 21 is generated as illustrated in FIG. 12C. The remaining portion 21 is turned to the overlap 22 when the remaining portion 21 is pushed by the roll during the step of roll-forming. If the overlap 22 is left as it is, it is likely to cause a crack due to a notch effect and the appearance will thus be inferior. Removing of the overlap 22 by machining will cause a large cost increase.